android: lowmemorykiller: modify return values to avoid extra calls

If the shrinker returns a large cache size, it will get called
repeatedly, proportional to the cache size divided by 128.  If
no processes to kill were found on the last pass through the
shrinker, there is no reason calling it again immediately will
find a process to kill.

Avoid the extra calls to the shrinker by:
Returning 0 when the shrinker is called with 0 and there are plenty
of free or file pages.  Avoids calling the shrinker at all during
this pass.
Returning rem when a thread is already dying.  Avoids telling the
shrinker core that the lowmemorykiller is making immense amounts
of forward progress.
Returning -1 when no process was selected for death.  Avoids calling
the shrinker again when nothing has changed.

Change-Id: I08f06063084f6070e71ac6afa7b14377149f3685
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>